Self-trimming wingsails of the general type to which this invention relates are variously described in applicant's earlier patent specifications such as U.S. Pat. No. 4,467,741 and U.S. Pat. No. 4,563,970. The wingsail basically consists of one or more thrust wings, each wing having a leading and trailing aerofoil section. The aerofoils are preferably of symmetrical cross-section. Previously the leading section has been mounted to the vessel for free rotation on an upright axis and the trailing section has been pivoted to the leading section about a spanwise axis so that it could be deflected from side to side to create mirror image cambered wing configurations for sailing on opposite tacks. An air directing slat has been positioned at the trailing edge of the leading section to define an aerodynamic slot between the leading section and the deflected trailing section.
The thrust wing or wings have been trimmed about the main upright axis by a tail aerofoil extending downstream from the wing.
With such an arrangement, the tail control has proved satisfactory at large angles of attack for providing high levels of thrust, but at lower angles of attack with the trailing section deflected, airflow through the slot has tended to separate, giving reduced control by the tail aerofoil which can become engulfed by disturbed airflow. The level of control worsens as the trust level is reduced, until at around the important zero thrust or neutral position, that is when zero crosswind force is developed, control may be lost altogether. Zero crosswind force means that there is zero thrust and no force is developed across the direction of the wind. The problem may be overcome in multi thrust wing wingsails as a single tail aerofoil may be offset between the two planes of, for example, a biwing wingsail. However the problem remains for monoplane thrust wings.